q.) If is an unbiased estimator of ,then which of the following is true:

(a) a + b is an unbiased estimator of a + b for all possible values of a and b.

(b) a + b is an unbiased estimator of a + b only when both a and b are strictly positive (c) a + b is an unbiased estimator of a + b only when a is strictly positive and b = 0.

(d) a + b is an unbiased estimator of a + b only when a is strictly positive and for all possible values of b